Peculiar lesions of mitral valves and papillary muscles developed in 50 to 80% of rabbits after clipping (N = 38), crushing (N = 15) or electrical stimulation (N = 44) of their cervical vagi. Both right and left vagus manipulations induced similar cardiac lesions. The earliest manifestation was bleeding of the mitral leaflets, which was followed by swelling and fibrosis of the papillary muscles. Ventricular arrhythmias and systolic murmurs were frequently associated with the vagus manipulations. During or just after electrical stimulation of the vagus, ventricular premature contractions (VPCs) were observed in 30 of 44 animals. Twenty-nine of these 30 animals with VPCs (96.7%) were found to have mitral valve and/or papillary muscle lesions at autopsy, whereas only 5 of 14 animals without VPCs (35.7%) had the cardiac lesions. These results indicate that the occurrence of these ventricular arrhythmias during vagus manipulations was closely related with the mitral valve and papillary muscle lesions.